The present invention relates to an improved jacking apparatus. More particularly, the present invention relates to an improved jacking apparatus for an offshore platform to effect relative movement between the platform and platform supporting legs.
Offshore platforms are customarily employed for supporting oil drilling equipment or servicing equipment above open expanses of water. It is desirable for the platform to be elevated above the water surface so as to be relieved of the effects of wave action. The platform is generally mounted on a plurality of supporting legs which are lowered to the water bed for support. Subsequently the platform is raised along the supporting legs to an elevated position above the water surface for effecting the necessary offshore operations. A jacking mechanism is employed on the platform for lowering the legs to the water bed and raising the platform along the legs, i.e., effecting relative vertical movement between the platform and the legs.
Numerous forms of jacking mechanisms or apparatus have been proposed. One such mechanism is disclosed in my earlier U.S. Pat. No. 3,604,683 issued Sept. 14, 1971. In this patent, the jacking mechanism combines the action of hydraulic cylinders with a power driven threaded screw to effect climbing relative to a leg of a platform. Hydraulic cylinders are mounted at one end to a suitable support secured to the platform and at the opposite end to a movable carrier head having a pin connection selectively engagable with the leg. The pin connection operates in conjunction with a track along which the carrier head travels. Movement of the carrier head is supplemented by the action of a threaded screw which acts primarily as a safety feature.
A further jacking mechanism is described in my earlier U.S. Pat. No. 3,517,910 issued June 30, 1970. In this arrangement, each jacking assembly includes a yoke spaced from the deck of the platform and slidably mounted upon a supporting leg. An engagement pin is provided on the yoke which pin can be selectively engaged within a bore provided in a solid rectangular track preferably welded to an outside surface of the leg. The pin must be extended outwardly a substantial distance from the yoke to engage the bore or recess in the track. With such an arrangement, there is a substantial bending moment upon the pin when the pin is engaged within the bore. At least one jack comprising a threaded rod of a ball screw type is extensible and retractable for moving the yoke.
Still a further jacking mechanism is disclosed in my prior U.S. Pat. No. 4,007,914 issued Feb. 15, 1977. The jacking apparatus comprises a frame which includes a cross member mounted on the platform. Vertically spaced first and second fluid cylinders are carried by the platform with the first cylinder being mounted on the cross member. Each fluid cylinder carries a beam and a holding device for selectively coupling the beam to the leg. The cylinders and the holding devices are operated 180.degree. out of phase to effect step-by-step movement of the platform relative to the leg. The holding device includes a hydraulically actuated pin which is adapted to fit within recesses or openings provided directly within the support leg of the platform. Due to the distance between the holding pin and the leg, and also the line contact between the holding pin and the recess in the leg, a substantial bending force is exerted upon the holding pin.
Further jacking mechanisms are disclosed in my other prior U.S. Pat. Nos. 3,804,369 issued Apr. 16, 1974 and 4,203,576 issued May 21, 1980.
In addition, it has been proposed to use a toothed rack secured along a longitudinal length of the leg. A rotatable pinion cooperates with the toothed rack to move the platform relative to the leg. It is submitted that such a rack and pinion arrangement requires a substantial expense in machining the teeth of the rack to withstand the high loads to which the teeth are subjected. In addition, it is submitted that such an arrangement requires a large amount of power to produce the desired amount of movement between the leg and the platform.
Notwithstanding the novel features and significant advantages of my previously patented systems, there remains room for improvement. In particular, it is desirable to produce a jacking mechanism which is both relatively simple in operation and comparatively inexpensive to produce. Further, it is desirable to provide a jacking mechanism which more efficiently distributes the forces generated during jacking whereby parts of the jacking mechanism can be manufactured from less expensive steels or other materials.
Accordingly, it is an object of the present invention to provide an improved jacking apparatus for effecting relative movement between a platform and supporting legs carried by the platform.
A further object of the present invention is to provide a jacking apparatus which can be operated substantially continuously with only minor interruptions during the jacking operation.
It is still a further object of the present invention to provide a jacking apparatus that reduces the stresses occurring on a holding mechanism for the jacking apparatus. In addition, it is an object of the present invention to reduce the forces exerted upon the supporting leg of the platform.
These objects and others are accomplished by a jacking apparatus according to the present invention for effecting relative vertical movement between an upright leg and a platform. The apparatus includes a frame for mounting on the platform which frame includes at least two generally vertical side rails. A carrier is mounted on the frame adjacent to the upright leg for vertical movement relative to the rails. Fluid cylinders operably connected between the platform and the carrier effect movement of the carrier relative to the rails. First and second elongated jack tracks fixedly secured in a generally vertical orientation to the upright leg are provided with each of the jack tracks being received within a correspondingly shaped recess in the carrier. Each of the jack tracks includes a plurality of regularly spaced openings extending completely through the jack track in a direction generally along the circumference of the leg. First and second holding mechanisms supported by the carrier are adapted to selectively engage an aligned one of the openings in the first and second jack tracks respectively to prevent vertical movement of the carrier relative to the leg. Each of the holding mechanisms has a width which exceeds a width of the opening in the jack track in the circumferential direction of the leg.
In the preferred embodiment, the first and second jack tracks are arranged at an angle of substantially 90.degree. relative to one another. Further in a preferred embodiment, each of the holding mechanisms comprises a pin movably mounted within a cylinder supported by the carrier. The cylinder includes the recess and extends to a location closely adjacent an outer circumferential surface of the leg. The sliding tolerences between the recesses in the carrier and the corresponding jack track is preferably very small.
In a first embodiment, the leg comprises an elongated generally cylindrical column having a longitudinal opening therein. One of the jack tracks is fixedly secured to each side of the opening. A reinforcement plate is preferably fixedly secured between the two jack tracks arranged within the opening for stability and to further strengthen the assembly.
In a further embodiment of the present invention, the two jack tracks are fixedly secured to one another and fixedly secured to an outside circumference of the leg. The jack tracks may be formed as a continuous member or, alternatively, may be comprised of a plurality of individual sections fixedly secured to an outside surface of the leg and forming a continuous uninterrupted vertical track along the leg.
Since the width of the pin is greater than the width of the opening in the jack track, a generally U-shaped shear line exists when the pin engages the jack track opening. In this way, the shear stress is distributed over a larger surface area and the bending moment on the pin is substantially reduced. Further, by arranging the cylinder for the holding pin in close proximity to the outer circumference of the leg, the bending moment on the pin is further reduced since the unsupported length of the pin is substantially eliminated. Still further, the cylinder surrounding the pin further reduces the bending moment on the pin by providing an upper surface against which the pin abuts.
According to a further aspect of the present invention, three fluid cylinders are operably connected between the platform and the carrier for effecting movement of the carrier relative to the rails. The three fluid cylinders are arranged to partially surround the leg such that imaginary lines joining centers of the cylinders form an isosceles triangle. The base of the triangle is bisected by an imaginary line passing through the apex of the triangle and a longitudinal axis of the leg such that the longitudinal axis does not lie outside the triangle. With this arrangement of the fluid cylinders, the carrier is more evenly balanced to permit easier movement of the carrier along the jack tracks which are closely received within the recesses within the carrier. Further, by providing the cylinders in the above described pattern, the bending moment on the holding pins is further reduced.
According to still a further aspect of the present invention, the fluid cylinders have an extensible and retractable piston disposed therein which piston moves in an extensible direction during both upward movement of the platform relative to the leg and downward movement of the leg relative to the platform. In still a further aspect of the present invention, the frame includes a generally horizontal cross member secured to the vertical side rails with the cross member being vertically spaced from the platform. Further fluid cylinders are operably connected between the cross bar and a further carrier for effecting movement of the further carrier relative to the rails. Third and fourth holding mechanisms supported by the carrier are provided for selectively engaging aligned ones of the openings in the jack tracks to prevent vertical movement of the further carrier relative to the leg.
The further fluid cylinders may be arranged to be selectively extensible and retractable in reverse phase relationship to the first cylinders. In this way, a substantially continuous step-wise jacking operation can be accomplished by the present invention. In other words, by alternating movement between the two carriers in sequence, the jacking operation can be undertaken substantially continuously. Alternatively, all of the fluid cylinders may be arranged to operate in unison to provide an increased lifting force. This increased force is particularly useful when pulling the legs from the water bed or when the platform is heavily loaded.